1. Field of the Invention
This invention relates to cold cathode ionization gauges and, in particular, to means for initiating the discharge in such gauges.
2. Discussion of the Prior Art
One of the gauges frequently used for measuring gas pressures in the high vacuum regime down to 10.sup.-10 Torr, and occasionally even to 10.sup.-14 Torr, is the cold cathode ionization gauge (CCG). A cold cathode discharge gauge utilizes a self sustaining discharge between an anode and cathode. A magnetic field suitably arranged forces the electrons into very long paths. There are several types of CCG with differing geometries: the Penning, the magnetron, and the inverted magnetron. These are discussed by A. Berman in his book, "Total Pressure Measurements in Vacuum Technology".
Starting the discharge in a CCG requires an initial chance ionizing event, such as ionization by cosmic rays or a field emitted electron. The delay between turning on the high voltage to a CCG and the beginning of the build up of current in the discharge (starting time) is dependent upon the pressure. For a typical gauge it may require seconds at 10.sup.-5 Torr, and several hours at 10.sup.-10 Torr. Thus, the starting delay at low pressures may be unacceptably long.
The problem of delayed starting in a CCG is well recognized, and is discussed by Berman on page 219 of the above-mentioned "Total Pressure Measurements in Vacuum Technology".
Berman discusses several techniques for initiating the discharge in a CCG. These are:
(1) providing a pulse of electrons from a hot filament; PA1 (2) incorporating a sharp point or edge on the cathode or anode to provide field emission electrons, or field ionization of residual gas; and PA1 (3) including a radioactive source to provide initial ionizing means. PA1 (4) using an external source of short wavelength light to provide electrons by photoemission.
In addition there is another method, not mentioned by Berman, which, to the best of applicants' knowledge, has never been used in a commercial gauge:
Method (1) is effected by including within the gauge a thermionic electron source to trigger the discharge. Triggering the discharge requires the operator to decide that conditions are such that a discharge should exist, but does not. Then it is necessary to briefly push a trigger button on the control panel to start the discharge. An annoying pressure burst accompanies heating of the trigger filament, and if the button is actuated at high pressures, the filament can be damaged.
Method (2) is characterized by the use of a sharp point or edge on one of the electrodes and is a technique presently used by Balzers ag. The problem with this method is that the sharp point or edge is dulled by the action of the discharge, or its emission characteristics are altered by the presence of films formed in the discharge. Although it can work for a time, it is not dependable over long gauge operating times.
Method (3), the use of a radioactive source, requires fairly large radioactive sources to provide adequate ionization for starting at low pressures. This method is disclosed in J. Vac. Sci. Technol 3, 286, (1966) by C. Hayashi; H. Mennenga and W. Schaedler in Proc. Fourth International Vac. Congress 1968, p. 656; and British Pat. No. 1535314 of B. D. Power and C. R. D. Priestland. With the present attitude toward the hazards of radioactive materials, this method is impractical today. The problem is particularly serious in CCG's because the source must be closely positioned to the discharge cell electrodes where the radioactive material is subject to dispersal. Alpha and beta rays have very little penetrating ability, and thus must be used as superficial sources. The discharge causes sputtering of the electrodes, so that the radioactive source material can be spread about the vacuum system. Another concern is that workers assembling such gauges would be exposed to the radiation.
With respect to method (4), Paul Redhead reported in Can. J. Physics, 37, 1260 (1959) that the discharge in a CCG can be started by an external UV light, see page 1266. The gauge had a glass envelope which allowed some light from the external UV source to enter the gauge, although glass is not very transparent to UV light. The energetic UV light freed photo electrons from the gauge cathode, aiding in starting the discharge.
The disadvantage of this method as practiced by Redhead is the requirement that the envelope be transparent--all commercial gauges today are made of metal--and the need for a sizable, expensive UV source.